Method to control silver concentration in a resistance variable memory element

ABSTRACT

A method for controlling silver doping of a chalcogenide glass in a resistance variable memory element is disclosed herein. The method includes forming a thin metal containing layer having a thickness of less than about 250 Angstroms over a second chalcogenide glass layer, formed over a first metal containing layer, formed over a first chalcogenide glass layer. The thin metal containing layer preferably is a silver layer. An electrode may be formed over the thin silver layer. The electrode preferably does not contain silver.

FIELD OF THE INVENTION

[0001] The invention relates to the field of random access memory (RAM) devices formed using a resistance variable material, and in particular to controlling silver incorporation into a resistance variable memory element formed using chalcogenide glass.

BACKGROUND OF THE INVENTION

[0002] A well-known semiconductor memory component is random access memory (RAM). RAM permits repeated read and write operations on memory elements. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Non-limiting examples of RAM devices include dynamic random access memory (DRAM), synchronized dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMS and SDRAMS also typically store data in capacitors, which require periodic refreshing to maintain the stored data.

[0003] Recently resistance variable memory elements have been investigated for suitability as semi-volatile and non-volatile random access memory elements. In a resistance variable memory element, a conductive material, such as silver, is incorporated into a dielectric material. The resistance of the conductive material containing dielectric material can be changed between high resistance and low resistance states. The resistance variable memory element is normally in a high resistance state when at rest. A write operation to a low resistance state is performed by applying a voltage potential across the two electrodes.

[0004] One preferred resistance variable material comprises a chalcogenide glass. A specific example is germanium-selenide (Ge_(X)Se_(100−x)) containing a silver (Ag) component. One method of providing silver to the germanium-selenide composition is to initially form a germanium-selenide glass and then deposit a thin layer of silver upon the glass, for example by sputtering, physical vapor deposition, or other known techniques in the art. The layer of silver is irradiated, preferably with electromagnetic energy at a wavelength less than 600 nanometers, so that the energy passes through the silver and to the silver/glass interface, to break a chalcogenide bond of the chalcogenide material such that the glass is doped with silver. Another method for providing silver to the glass is to provide a layer of silver-selenide on a germanium-selenide glass. A top electrode comprising silver is then formed over the silver-germanium-selenium glass or in the case where a silver selenide layer is provided over a germanium-selenide glass, the top electrode is formed over the silver-selenide layer.

[0005] It has been found that over time devices fabricated via the above described methods fail if excess silver from a top silver containing electrode continues to diffuse into the silver germanium-selenium glass or into the silver-selenide layer and eventually into the germanium-selenide glass layer (the primary switching area) below the silver-selenide layer.

[0006] Furthermore, during semiconductor processing and/or packaging of a fabricated structure, which incorporates the memory element, the structure undergoes thermal cycling or heat processing. The memory element is also subject to heat during operation of the memory device containing the memory element. Heat processing can result in substantial amounts of silver migrating into the memory element uncontrollably. Too much silver incorporated into the memory element may result in faster degradation, i.e., a short life, and eventually device failure.

[0007] Control of the amount of available silver that enters the glass would be highly desirable to prevent premature memory cell failure.

BRIEF SUMMARY OF THE INVENTION

[0008] An exemplary embodiment of the present invention includes a method of fabricating a resistance variable memory element and the resulting device wherein silver doping of a chalcogenide glass is controlled. The method includes forming a first chalcogenide glass layer; forming a first silver-selenide layer in contact with the first chalcogenide glass layer; forming a second chalcogenide glass layer in contact with the first silver-selenide layer; and forming a thin silver layer in contact with the second chalcogenide glass layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other features and advantages of the invention will be better understood from the following detailed description, which is provided in connection with the accompanying drawings.

[0010]FIG. 1 illustrates a cross-sectional view of a memory element fabricated in accordance with a first embodiment of the invention and at an initial stage of processing.

[0011]FIG. 2 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 1.

[0012]FIG. 3 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 2.

[0013]FIG. 4 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 3.

[0014]FIG. 5 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 4.

[0015]FIG. 6 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 5.

[0016]FIG. 7 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 6.

[0017]FIG. 8 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 7.

[0018]FIG. 9 illustrates a cross-sectional view of the memory element of FIG. 1 at a stage of processing subsequent to that shown in FIG. 8.

[0019]FIG. 10 illustrates a processor-based system having a memory element formed according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] In the following detailed description, reference is made to various specific embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made without departing from the spirit or scope of the invention.

[0021] The term “substrate” used in the following description may include any supporting structure including, but not limited to, a plastic or a semiconductor substrate that has an exposed substrate surface. A semiconductor substrate should be understood to include silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor material structures. When reference is made to a semiconductor substrate or wafer in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation.

[0022] The term “silver” is intended to include not only elemental silver, but silver with other trace metals or in various alloyed combinations with other metals as known in the semiconductor industry, as long as such silver alloy is conductive, and as long as the physical and electrical properties of the silver remain unchanged.

[0023] The term “silver-selenide” is intended to include various species of silver-selenide, including some species, which have a slight excess or deficit of silver, for instance, Ag₂Se, Ag_(2+x)Se, and Ag_(2−x)Se.

[0024] The term “chalcogenide glass” is intended to include glasses that comprise an element from group VIA (or group 16) of the periodic table. Group VIA elements, also referred to as chalcogens, include sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and oxygen (O).

[0025] The invention will now be explained with reference to FIGS. 1-10, which illustrate exemplary embodiments of a resistance variable memory element 100 fabricated in accordance with the invention. FIG. 1 depicts a portion of an insulating layer 12 formed over a semiconductor substrate 10, for example, a silicon substrate. As noted earlier, it should be understood that the resistance variable memory element can be formed on a variety of substrate materials and not just semiconductor substrates. The insulating layer 12 may be formed by any known deposition methods, such as sputtering by chemical vapor deposition (CVD), plasma enhanced CVD (PECVD) or physical vapor deposition (PVD). The insulating layer 12 may be formed of a conventional insulating oxide, such as silicon oxide (SiO₂), a silicon nitride (Si₃N₄), or a low dielectric constant material, among many others.

[0026] A first electrode layer 14 is next formed over the insulating layer 12, as also illustrated in FIG. 1. The first electrode layer 14 may comprise any conductive material, for example, tungsten, nickel, tantalum, aluminum, or platinum, among many others. A first dielectric layer 15 is next formed over the first electrode 14. The first dielectric layer 15 may comprise the same or different materials as those described above with reference to the insulating layer 12.

[0027] Referring now to FIG. 2, an opening 13 extending to the first electrode layer 14 is formed in the first dielectric layer 15. The opening 13 may be formed by known methods in the art, for example, by a conventional patterning and etching process. A first chalcogenide glass layer 17 is next formed over the first dielectric layer 15, to fill in the opening 13, as shown in FIG. 3.

[0028] According to a first embodiment of the invention, the first chalcogenide glass layer 17 is a germanium-selenide glass having a Ge_(x)Se_(100—x) stoichiometry. The preferred stoichiometric range is between about Ge₁₈Se₈₂ to about Ge₄₃Se₅₇ and is more preferably about Ge₄₀Se₆₀. The first chalcogenide glass layer 17 preferably has a thickness from about 100 Å to about 1000 Å and is more preferably about 150 Å.

[0029] The use of a metal containing layer, such as a silver-selenide layer, in contact with the chalcogenide glass layer 17 makes it unnecessary to photodope the glass with silver. However, it is possible to also metal (e.g., silver) dope the chalcogenide glass layer, which is in contact with the silver-selenide layer, as an optional variant.

[0030] The formation of the first chalcogenide glass layer 17, having a stoichiometric composition in accordance with the invention may be accomplished by any suitable method. For instance, evaporation, co-sputtering germanium and selenium in the appropriate ratios, sputtering using a germanium-selenide target having the desired stoichiometry, or chemical vapor deposition with stoichiometric amounts of GeH₄ and SeH₂ gases (or various compositions of these gases), which result in a germanium-selenide film of the desired stoichiometry are examples of methods which may be used to form the first chalcogenide glass layer 17.

[0031] Referring now to FIG. 4, a first metal containing layer 18, preferably silver-selenide, is deposited over the first chalcogenide glass layer 17. Any suitable metal containing layer may be used. For instance, other suitable metal containing layers include silver-chalcogenide layers. Silver sulfide, silver oxide, and silver telluride may be suitable silver-chalcogenides for use in combination with any suitable chalcogenide glass layer 17. For purposes of simplified discussion, metal containing layer 18 will be further described herein as a silver selenide layer. However, it should be understood that other metal containing layers, including those just identified could also be used. A variety of processes can be used to form the silver-selenide layer 18. For instance, physical vapor deposition techniques such as evaporative deposition and sputtering may be used. Other processes such as chemical vapor deposition, co-evaporation or depositing a layer of selenium above a layer of silver to form silver-selenide can also be used.

[0032] The layers 17 and 18 may be any suitable thickness depending upon the mechanism for switching. Preferably, the thickness of the layers is such that the silver-selenide layer 18 is thicker than the first chalcogenide glass layer 17. The silver-selenide layer 18 is also thicker than a second chalcogenide glass layer, described below. More preferably, the thickness of the layers are such that a ratio of the silver-selenide layer thickness to the first chalcogenide glass layer thickness is between about 5:1 and about 1:1. In other words, the thickness of the silver-selenide layer 18 is between about 1 to about 5 times greater than the thickness of the first chalcogenide glass layer. Even more preferably, the ratio is between about 3.1:1 and about 2:1 silver-selenide layer thickness to first chalcogenide glass layer thickness.

[0033] Referring now to FIG. 5 a second chalcogenide glass layer 20 is formed over the silver-selenide layer 18. The second glass layer 20 allows deposition of silver above the silver-selenide layer 18, since silver cannot be directly sputtered on silver-selenide.

[0034] The second chalcogenide glass layer 20 is preferably a germanium-selenide glass having a Ge_(X)Se_(100−X) stoichiometry. The second chalcogenide glass layer 20 may, but need not, have the same stoichiometric composition as the first chalcogenide glass layer. However, the preferred stoichiometric range is the same as the first glass layer and thus is between about Ge₁₈Se₈₂ to about Ge₄₃Se₅₇ and is more preferably about Ge₄₀Se_(60.)

[0035] The thickness of the layers are such that the silver-selenide layer 18 thickness is greater than the thickness of the second chalcogenide glass layer 20. Preferably, a ratio of the silver-selenide layer 18 thickness to the second chalcogenide glass layer 20 thickness is between about 5:1 and about 1:1. More preferably, the ratio of the silver-selenide layer 18 thickness to the thickness of the second chalcogenide glass layer 20 is between about 3.3:1 and about 2:1 silver-selenide layer 18 thickness to second chalcogenide glass layer 20 thickness. The second chalcogenide glass layer 20 preferably has a thickness between about 100 Å to about 1000 Å and is more preferably 150 Å.

[0036] The formation of the second chalcogenide glass layer 20 may be accomplished by any suitable method. For instance, chemical vapor deposition, evaporation, co-sputtering, or sputtering using a target having the desired stoichiometry, may be used.

[0037] Referring now to FIG. 6, silver layer 22 is formed over the second glass layer 20. The silver layer is preferably between about 50 Å to about 250 Å thick and is more preferably 200 Å thick. The thickness of the second chalcogenide glass layer 20 and the silver layer 22 are preferably such that sufficient silver is available to ensure the resistance between the silver selenide layer 18 and a top electrode (to be described later) is sufficiently low. The use of too much silver, for example, a silver layer having a thickness of greater than about 250 Å limits the functionality of the device at high temperature processing, for example, of about 200° C. Devices having a thin layer of silver (for example, 50 Å) last substantially longer in high temperatures (for example, 200° C.). The silver layer may be deposited by any suitable mechanism, for instance, physical vapor deposition (PVD) or evaporation, however, sputter deposition is preferred.

[0038] Referring now to FIG. 7, a second conductive electrode material 24 is formed over the silver layer 22. The second conductive electrode material 22 may comprise any electrically conductive material, for example, tungsten, tantalum or titanium, among many others, but does not contain silver.

[0039] Referring now to FIG. 8, an optional tungsten nitride layer 26 may be formed over the second electrode material 24.

[0040] Referring now to FIG. 9, one or more additional dielectric layers 30 may be formed over the second electrode 24 or alternatively over the tungsten nitride layer 26 and the first dielectric layer 15 (as shown) to isolate the resistance variable memory element 100 from other structures fabricated over the substrate. Conventional processing steps can then be carried out to electrically couple the second electrode 24 to various circuits of memory arrays.

[0041] Devices constructed according to the invention, particularly those incorporating a thin layer of silver, having a thickness of between about 50 Å to about 250 Å, underlying an uppermost electrode, shows improved functionality at higher processing temperatures, thereby allowing devices in accordance with the invention to be processed at higher temperatures. Devices according to the invention also show improved functionality at high operating temperatures thus devices according to the invention are more durable. Accordingly, a device in accordance with the invention shows improved durability over conventional resistance variable memory devices.

[0042]0 Although the embodiments described above refer to the formation of only one resistance variable memory element 100, it must be understood that the invention contemplates the formation of any number of such resistance variable memory elements, which can be fabricated in a memory array and operated with memory element access circuits.

[0043]FIG. 10 illustrates an exemplary processing system 900, which utilizes a resistance variable memory element constructed as described above in a memory device 101. The processing system 900 includes one or more processors 901 coupled to a local bus 904. A memory controller 902 and a primary bus bridge 903 are also coupled the local bus 904. The processing system 900 may include multiple memory controllers 902 and/or multiple primary bus bridges 903. The memory controller 902 and the primary bus bridge 903 may be integrated as a single device 906.

[0044] The memory controller 902 is also coupled to one or more memory buses 907. Each memory bus accepts memory components 908, which include at least one memory device 101 of the invention. Alternatively, in a simplified system, the memory controller 902 may be omitted and the memory components directly coupled to one or more processors 901. The memory components 908 may be a memory card or a memory module. The memory components 908 may include one or more additional devices 909. For example, the additional device 909 might be a configuration memory. The memory controller 902 may also be coupled to a cache memory 905. The cache memory 905 may be the only cache memory in the processing system. Alternatively, other devices, for example, processors 901 may also include cache memories, which may form a cache hierarchy with cache memory 905. If the processing system 900 include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller 902 may implement a cache coherency protocol. If the memory controller 902 is coupled to a plurality of memory buses 907, each memory bus 907 may be operated in parallel, or different address ranges may be mapped to different memory buses 907.

[0045] The primary bus bridge 903 is coupled to at least one peripheral bus 910. Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus 910. These devices may include a storage controller 911, a miscellaneous I/O device 914, a secondary bus bridge 915, a multimedia processor 918, and a legacy device interface 920. The primary bus bridge 903 may also coupled to one or more special purpose high speed ports 922. In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system 900.

[0046] The storage controller 911 couples one or more storage devices 913, via a storage bus 912, to the peripheral bus 910. For example, the storage controller 911 may be a SCSI controller and storage devices 913 may be SCSI discs. The I/O device 914 may be any sort of peripheral. For example, the I/O device 914 may be a local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be an universal serial port (USB) controller used to couple USB devices 917 via to the processing system 900. The multimedia processor 918 may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional devices such as speakers 919. The legacy device interface 920 is used to couple legacy devices, for example, older styled keyboards and mice, to the processing system 900.

[0047] The processing system 900 illustrated in FIG. 10 is only an exemplary processing system with which the invention may be used. While FIG. 10 illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system 900 to become more suitable for use in a variety of applications. For example, many electronic devices, which require processing may be implemented using a simpler architecture that relies on a CPU 901, coupled to memory components 908 and/or memory elements 101. These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.

[0048] The invention is not limited to the details of the illustrated embodiment. Accordingly, the above description and drawings are only to be considered illustrative of exemplary embodiments which achieve the features and advantages of the invention. Modifications and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims. 

What is claimed as new and desired to be protected by Letters Patent of the United States is:
 1. A method of forming a resistance variable memory element comprising: forming a first chalcogenide glass layer; forming a first metal containing layer in contact with said first chalcogenide glass layer; forming a second chalcogenide glass layer in contact with said first metal containing layer; and forming a second metal containing layer having a thickness of less than about 250 Å in contact with said first chalcogenide glass layer.
 2. The method of claim 1 wherein said first chalcogenide glass layer comprises a material having the formula Ge_(x)Se_(100−x), wherein x is between about 18 to about
 43. 3. The method of claim 2 wherein said first chalcogenide glass layer has a stoichiometry of about Ge₄₀Se₆₀.
 4. The method of claim 1 wherein said first chalcogenide glass layer has a thickness from about 100 Å to about 1000 Å.
 5. The method of claim 4 wherein said first chalcogenide glass layer has a thickness of about 150 Å.
 6. The method of claim 1 wherein said first metal containing layer has a thickness about 1 to about 5 times greater than the thickness of the first chalcogenide glass layer.
 7. The method of claim 1 wherein said second chalcogenide glass layer comprises a material having the formula Ge_(x)Se_(100−x), wherein x is between about 18 to about
 43. 8. The method of claim 7 wherein said second chalcogenide glass layer has a stoichiometry of Ge₄₀Se₆₀.
 9. The method of claim 1 wherein said second chalcogenide glass layer has a thickness of from about 100 Å to about 1000 Å.
 10. The method of claim 9 wherein said second chalcogenide glass layer has a thickness of about 150 Å.
 11. The method of claim 1 wherein said second metal containing layer comprises a silver layer.
 12. The method of claim 1 wherein said second metal containing layer has a thickness between about 50 Å to about 250 Å.
 13. The method of claim 12 wherein said second metal containing layer has a thickness of about 200 Å.
 14. The method of claim 1 wherein said first metal containing layer comprises silver-selenide.
 15. The method of claim 1 wherein said second metal containing layer comprises a silver layer.
 16. The method of claim 1 comprising forming a first electrode under said first chalcogenide glass layer.
 17. The method of claim 16 comprising forming a second electrode overlying and in electrical communication with said metal containing glass layer.
 18. The method of claim 17 wherein said second electrode is not formed of silver.
 19. A method of forming a resistance variable memory element comprising the steps of: forming a first electrode; forming a first germanium-selenium glass layer over said first electrode; forming a first silver-selenide layer over said first germanium-selenium glass layer; forming a second germanium-selenium glass layer over said first silver-selenide layer; forming a silver containing layer having a thickness of less than about 250 Å over said second germanium-selenium glass layer; and forming a second electrode overlying and in electrical communication with said silver containing layer.
 20. The method of claim 19 wherein said first germanium-selenium glass layer comprises a material having the formula Ge_(x)Se_(100−x), wherein x is between about 18 to about
 43. 21. The method of claim 20 wherein said first germanium-selenium glass layer has a stoichiometry of about Ge₄₀Se₆₀.
 22. The method of claim 19 wherein said first germanium-selenium glass layer has a thickness from about 100 Å to about 1000 Å.
 23. The method of claim 22 wherein said first germanium-selenium glass layer has a thickness of about 150 Å.
 24. The method of claim 19 wherein said first silver-selenide layer has a thickness about 1 to about 5 times greater than the thickness of said first germanium-selenium glass layer.
 25. The method of claim 19 wherein said second germanium-selenium glass layer comprises a material having the formula Ge_(x)Se_(100−x), wherein x is between about 18 to about
 43. 26. The method of claim 25 wherein said second germanium-selenium glass layer has a stoichiometry of about Ge₄₀Se_(60.)
 27. The method of claim 19 wherein said second germanium-selenium glass layer has a thickness of from about 100 Å to about 1000 Å.
 28. The method of claim 26 wherein said second germanium-selenium glass layer has a thickness of about 150 Å.
 29. The method of claim 19 wherein said silver containing layer has a thickness between about 50 Å to about 250 Å.
 30. The method of claim 29 wherein said silver containing layer has a thickness of about 200 Å.
 31. The method of claim 19 wherein said second electrode is not formed of silver.
 32. A resistance variable memory element comprising: a first chalcogenide glass layer; a first metal containing layer overlying said first chalcogenide glass layer; a second chalcogenide glass layer overlying said first metal containing layer; and a second metal containing layer having a thickness of less than about 250 Å overlying said second chalcogenide glass layer.
 33. The memory element of claim 32 wherein said first metal containing layer comprises silver-selenide.
 34. The memory element of claim 32 wherein said second metal containing layer comprises a silver layer.
 35. The memory element of claim 32 wherein at least one of said first and said second chalcogenide glass layer comprises a material having the formula Ge_(x)Se_(100−x), wherein x=18 to
 43. 36. The memory element of claim 35 wherein at least one of said first and said second chalcogenide glass layers have a stoichiometry of about Ge₄₀Se₆₀.
 37. The memory element of claim 32 wherein said at least one of said first and said second glass layers have a thickness between about 100 Å and about 1000 Å.
 38. The memory element of claim 37 wherein at least one of said first and said second glass layers have a thickness of about 150 Å.
 39. The memory element of claim 32 wherein said first metal containing layer has a first thickness and said second chalcogenide glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 5:1 to about 1:1.
 40. The memory element of claim 39 wherein said first metal containing layer has a first thickness and said second chalcogenide glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 3.3:1 to about 2:1.
 41. The memory element of claim 32 wherein said first metal containing layer has a first thickness and said first glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 5:1 to about 1:1.
 42. The memory element of claim 41 wherein said first metal containing layer has a first thickness and said first glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 3.3:1 to about 2:1.
 43. A processor based system comprising the memory element of claim
 32. 44. A memory element comprising: a first electrode; a first germanium-selenium glass layer overlying and in electrical communication with said first electrode; a silver-selenide layer overlying and in electrical communication with said first germanium-selenium glass layer; a second germanium-selenium glass layer overlying and in electrical communication with said silver-selenium layer; a silver containing layer overlying and in electrical communication with said second germanium-selenium glass layer; and a second electrode overlying and in electrical communication with said silver containing layer.
 45. The memory element of claim 44 wherein said first and said second germanium-selenium glass layer has a stoichiometry of from about Ge₁₈Se₈₂ to about Ge₄₃Se₅₇.
 46. The memory element of claim 45 wherein at least one of said first and said second germanium-selenium glass layers has a stoichiometry of about Ge₄₀Se₆₀.
 47. The memory element of claim 46 wherein said silver containing layer has a thickness of between about 50 Å to about 250 Å.
 48. The memory element of claim 47 wherein said silver containing layer has a thickness of about 200 Å.
 49. The memory element of claim 44 wherein at least one of said first and said second germanium-selenium glass layer has a thickness between about 100 Å and about 1000 Å.
 50. The memory element of claim 49 wherein at least one of said first and said second germanium-selenium glass layers has a thickness of about 150 Å.
 51. The memory element of claim 44 wherein said silver selenide layer has a first thickness and said first germanium-selenium glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 5:1 to about 1:1.
 52. The memory element of claim 51 wherein said silver-selenide layer has a first thickness and said first germanium-selenium glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 3.3:1 to about 2:1.
 53. The memory element of claim 44 wherein said silver-selenide layer has a first thickness and said second germanium-selenium glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 5:1 to about 1:1.
 54. The memory element of claim 53 wherein said silver-selenide layer has a first thickness and said second germanium-selenium glass layer has a second thickness whereby a thickness ratio of said first thickness to said second thickness is between about 3.3:1 to about 2:1.
 55. The memory element of claim 44 wherein said second electrode does not comprise silver.
 56. A processor based system comprising the memory element of claim
 44. 